Affordable CO2 sensor?

So I stumbled upon this CO2 sensor manufacturer by googling.
I wrote to them explaining our hobby and we could use a sensitive sensor to turn on and off our CO2. I got a reply from them. See if this could work out. My main concern is the error range.

It uses infrared to read CO2 in the atmosphere unlike the less reliable chemical reaction sensors we're used too.

Quote:

Thanks for the interest and inquiry.
We measure the concentration of CO2 in water for example by measure the head space gas.
This make use of Henrys gas law regarding concentration in air and liquid.
The measurement are made via a space cup or a gas permeable membrane.
You could do this using a simple k30 sensor;http://www.co2meter.com/collections/...-sensor-module
I’m not familiar with the operation of aquarium but 30 PPM of CO2 in water sounds extremely low for water standing a state of equilibrium.

There must be a relationship between ppm in the water, and ppm in the air trapped above the water. I assume the partial pressures of the CO2 would be equal, but what would the relationship be between the two ppm's?

There must be a relationship between ppm in the water, and ppm in the air trapped above the water. I assume the partial pressures of the CO2 would be equal, but what would the relationship be between the two ppm's?

Given this diagram, I would imagine the CO2 levels would be the same as in the water.

For example, in the case of CO2 injection, the levels of CO2 in the water are higher. CO2 will off gas from the water into the atmosphere and into the bell shaped jar. When the levels of CO2 in the jar reaches the same level as the water, then it will not increase anymore.

Of course, CO2 will be diffusing from the water into the atmosphere as well as into the jar.

Given this diagram, I would imagine the CO2 levels would be the same as in the water.

For example, in the case of CO2 injection, the levels of CO2 in the water are higher. CO2 will off gas from the water into the atmosphere and into the bell shaped jar. When the levels of CO2 in the jar reaches the same level as the water, then it will not increase anymore.

Of course, CO2 will be diffusing from the water into the atmosphere as well as into the jar.

As I understand it, we use the term "ppm", but, really, that's technically totally inaccurate - we do *NOT* aim for 30 parts per million. And the CO2 solubility of water is far less than that of air.

And that's as much as I know about water chemistry, and I'm sure I explained it badly. But there are some posts on it, written by people how actually know what they're talking about.

This is an interesting gizmo that can be used for lots of things but for me it is not going to be an affordable option. It is not a stand alone item to plug in. It needs several other things before we can stick it in to measure the CO2 in tanks. Those other items will run the price way beyond the level most will pay. With mounts, power supply and connections, I see at least $200. With equipment to read concentrations in water rather than air as this is designed it will be out of my range for sure.

This is an interesting gizmo that can be used for lots of things but for me it is not going to be an affordable option. It is not a stand alone item to plug in. It needs several other things before we can stick it in to measure the CO2 in tanks. Those other items will run the price way beyond the level most will pay. With mounts, power supply and connections, I see at least $200. With equipment to read concentrations in water rather than air as this is designed it will be out of my range for sure.

Yeah but compared to ph controller ( not accurate) or a scientific water co2 sensor, it's a steal.

Given this diagram, I would imagine the CO2 levels would be the same as in the water.

For example, in the case of CO2 injection, the levels of CO2 in the water are higher. CO2 will off gas from the water into the atmosphere and into the bell shaped jar. When the levels of CO2 in the jar reaches the same level as the water, then it will not increase anymore.

Of course, CO2 will be diffusing from the water into the atmosphere as well as into the jar.

But, 30 ppm of CO2 dissolved in water isn't in equilibrium with 30 ppm of CO2 in air. My education is lacking in this field, so I have no idea how to even start to solve this.

Air can have a varying amount of CO2 in it. If it is .03% CO2, it is 300 ppm, but water exposed to that air will contain more like 3 ppm of CO2. If the ppm in the water in equilibrium with the air above the water is a near constant percentage of the ppm in the air, this is an easy problem, but Murphy's Law seems to dictate that it isn't anywhere near a constant percentage.

I am curious as to why a pH controller would not be accurate (I assume due to probe calibrations, etc)?

Same reason not to to use tank water in a drop checker. The ph will be affected by other things besides co2 throughout the day like rocks, substrate, driftwood. We're not dealing with an empty tank of water.

If the ppm in the water in equilibrium with the air above the water is a near constant percentage of the ppm in the air, this is an easy problem, but Murphy's Law seems to dictate that it isn't anywhere near a constant percentage.

Let's hope it's Henry's law, not Murphy's law that applies here!

My concern would be the speed of response... if the CO2 rises in the water, how fast will it equalize in the (non-moving) air space and turn off the solenoid?

But, 30 ppm of CO2 dissolved in water isn't in equilibrium with 30 ppm of CO2 in air. My education is lacking in this field, so I have no idea how to even start to solve this.

Air can have a varying amount of CO2 in it. If it is .03% CO2, it is 300 ppm, but water exposed to that air will contain more like 3 ppm of CO2. If the ppm in the water in equilibrium with the air above the water is a near constant percentage of the ppm in the air, this is an easy problem, but Murphy's Law seems to dictate that it isn't anywhere near a constant percentage.

So it is an issue with the rate of diffusion, but also the solubility of CO2 in the water?

I'm a microbiologist, not a chemist (though I have a minor, haha). Perhaps we need to get a chemist on this

Quote:

Originally Posted by mistergreen

Same reason not to to use tank water in a drop checker. The ph will be affected by other things besides co2 throughout the day like rocks, substrate, driftwood. We're not dealing with an empty tank of water.

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